There has conventionally been known, as a display of rear projection type (rear projection type television), a projection display using three CRTs of red, green and blue colors as an imaging light source, in which imaging light emitted from such an imaging light source is projected on the back surface of a transmission projection screen to produce an image, which is viewed from the viewer's side.
A projection screen for use in such a projection display is usually composed of a Fresnel lens sheet and a lenticular lens sheet, and allows imaging light emitted from an imaging light source to form an image and to emerge toward viewers as directional diffused light.
Specifically, for example, a projection screen 300 comprises, as shown in FIG. 20, a Fresnel lens sheet 301 having a circular Fresnel lens 302 formed on its emergent side surface, and, on the viewer's side of the Fresnel lens sheet 301, a lenticular lens sheet 303 having a lenticular lens 304 for horizontal diffusion formed on its incident side surface. On the emergent side surface of the lenticular lens sheet 303 are provided lenses 305 from which light emerges and black stripes 306.
Of these, the Fresnel lens 302 on the Fresnel lens sheet 301 can be obtained by grooving a transparent resin material, such as an acrylic resin, at a predetermined angle and at a predetermined pitch, and has the function of condensing, toward the viewer's side, radially diffused imaging light that is emitted from an imaging light source (not shown in the figure) placed at the rear of the Fresnel lens sheet 301. The lenticular lens 304 on the lenticular lens sheet 303 can be obtained by forming cylindrical unit lenses so that they extend on one plane regularly and longitudinally, and has the function of diffusing, chiefly in the horizontal direction, the imaging light condensed by the Fresnel lens sheet 301 to let the light emerge as directional diffused light in the horizontal direction.
In the meantime, in place of the above-described projection display using three CRTs of red, green and blue colors, a projection display of single lens mode which uses a cellular-structured imaging light source such as an LCD or DMD and in which imaging light emitted from such an imaging light source is projected on the back surface of a transmission projection screen, the produced image being viewed from the viewer's side, has been increasingly demanded in recent years.
Heretofore, the projection mode usually adopted in such a projection display of single lens mode is that imaging light is projected on the projection screen from its rear almost vertically to the projection screen. The drawback of a projection display of this mode has been that since it requires a depth nearly equal to that of a conventional CRT projection display, it cannot be made smaller.
Under these circumstances, there has been proposed, as one of projection displays, a projection display in which imaging light emitted from an imaging light source is obliquely projected on a projection screen in order to make the display considerably smaller than conventional ones without impairing image quality (see Japanese Laid-Open Patent Publications No. 208041/1986 and No. 180967/2000).
Such a projection display has, on its incident side surface, a group of unit prisms with triangular cross sections (total reflection prism lens) as an optical means of condensing imaging light obliquely incident on the projection screen; the first plane (plane of incidence) of each unit prism refracts the incident imaging light, and the second plane (plane of total reflection) of the unit prism then totally reflects the refracted light to let the reflected light emerge from the emergent side surface of the projection display.
In the projection screen having such a total reflection prism lens, in the area on the side close to the imaging light source (in the case where the unit prisms concentrically extend around the center of the concentric circles that is not on the screen plane, in the area on the side close to this center of the concentric circles) in which the angle of incidence of imaging light (the angle of imaging light with the screen plane) gets smaller, part of imaging light incident on the plane of incidence 311a of each unit prism 311 of the total reflection prism lens 310 is not totally reflected at the plane of total reflection 311b of the unit prism 311 and, as shown in FIG. 21, passes through this plane to become stray light, causing such troubles as the formation of double image (ghost). In FIG. 21, reference character L11 denotes the light path of a component of imaging light that becomes ordinary light, and reference character L12 denotes the light path of a component of imaging light that becomes stray light. The amount of stray light thus produced is greater when each unit prism 311 has a larger apical angle λ, and is smaller when each unit prism 311 has a smaller apical angle λ.
On the other hand, in the projection screen having the above-described total reflection prism lens, in the area on the side distant from the imaging light source (in the case where the unit prisms concentrically extend around the center of the concentric circles that is not on the screen plane, in the area on the side distant from this center of the concentric circles) in which the angle of incidence of imaging light is great, each unit prism 311 has a smaller apical angle λ, and its plane of incidence 311a gets reverse tapered, as shown in FIG. 22. Therefore, there has been such a problem that part of imaging light incident on the plane of incidence 311a of each unit prism 311 is totally reflected at the plane of total reflection 311b of the unit prism 311 and is then reflected again at the plane of incidence 311a to become stray light, causing imaging light loss. In FIG. 22, reference character L21 denotes the light path of a component of imaging light that becomes ordinary light, and reference character L22 denotes the light path of a component of imaging light that becomes stray light. Further, there has been such a problem that, if the planes of incidence 311a of the unit prisms 311 are reverse tapered, it becomes difficult to make a mold for use in the molding of the unit prisms 311 and also to release the unit prisms from the mold in the lens molding process. Furthermore, in the case where the mold for use in the molding of the unit prisms 311 is produced by cutting a mold material, it is difficult to shape the mold correspondingly to the reverse tapered planes of incidence 311a of the unit prisms 311, and, moreover, the planes of incidence 311a of the unit prisms 311 become rough surfaces with flaws created in the course of cutting. A problem with this case has been as follows: both the area in which the planes of incidence 311a of the unit prisms 311 are mirror surfaces and the area in which the planes of incidence 311a of the unit prisms 311 are rough surfaces are to exist on the screen plane, so that the image produced on the screen plane appears differently at the boundary between these two areas and is thus observed as being uneven.
Thus, the conventional projection screens have the following drawback: since they have narrow allowable ranges of the angle of incidence of imaging light and tend to cause imaging light loss due to the production of stray light or the like, they easily undergo lowering of surface brightness or contrast.